Rectally absorbable form of L-dopa

ABSTRACT

The invention relates to compositions and methods of enhancing rectal absorption of L-dopa via the formation of an ester prodrug and optionally with a decarboxylase inhibitor.

BACKGROUND OF THE INVENTION

This is a continuation-in-part of U.S. application Ser. No. 814,861 filed Dec. 30, 1985.

The invention relates to compositions and methods of use of ester prodrug derivatives of L-dopa for enhancing absorption via rectal therapeutic application. More specifically, the invention relates to alkyl, aryl, substituted and unsubstituted mono-, di- and polyhydroxyalkyl, and substituted and unsubstituted aralkyl esters of L-dopa and their pharmaceutically acceptable counterion salts. Also, the heretofore disclosed compositions and methods may optionally include a decarboxylase inhibitor such as carbidopa and benserazide. The inhibitor serves three vital roles, namely, 1.) prevents the breakdown of L-dopa, 2.) reduces commonly know side effects, and 3.) allows further expression of L-dopa pharmacological activity.

L-dopa is usually orally given with a L-amino acid decarboxylase inhibitor. Given in this fashion, L-dopa is effective in the treatment of Parkinson's disease. However, presumably due to erratic absorption and blood levels, control of disease symptoms is not always adequate. L-dopa esters have not provided significant advantages when given orally. This is probably due to ester hydrolysis in the g.i. tract and/or presystemic metabolism. These problems are ameliorated by the rectal route.

As employed in this application, the expression "prodrug" denotes a derivative of a known and proven prior art compound, which derivative, when absorbed into the bloodstream of a warm-blooded animal, "cleaves" in such a manner as to release the proven drug form and permits the same to afford improved therapeutic efficacy than that which could be obtained if the proven drug form, per se, was administered.

Also as employed in this application, the term "cleavage" denotes that the proven drug form is released and the remaining "cleaved" moiety is nontoxic and metabolized in such a manner that non-toxic, metabolic products are produced.

DESCRIPTION OF THE PRIOR ART

It is known to the art that dopa, L-dopa and dopamine, and their salts, are useful active agents for the treatment or management of a wide variety of disease states or conditions, e.g., they are useful anticholinergic agents, antiparkinsonian agents, adrenergic agents, cardiotonic agents, and the like. See generally Cutting's Handbook of Pharmacology, 41, "Sympathetic Stimulants of Adrenergic Agents", pp. 436-455, Sixth Edition (1979); The Merck Index, pp. 3424 & 5314, Ninth Edition (1976).

Nevertheless, it is also known to the art that sympathomimetic amines and various art-recognized therapeutically active derivatives thereof, are characterized by certain inherent disadvantages, notably serious bioavailability and physiological availability problems upon administration. Such reduced availability can be attributed in part to poor lipid solubility [by reason of the presence of the hydrophilic phenolic hydroxyl groups], and also to metabolic losses during and following conventional administration.

Also, it is further known to the art that L-dopa, following oral administration, undergoes extensive metabolism within the stomach and small intestine. Esters of L-dopa, when given orally, offer no advantage over L-dopa itself. Additionally, certain portions of the patient population experience extreme difficulty in swallowing oral formulations.

SUMMARY OF THE INVENTION

The invention relates to compositions and methods of enhancing rectal absorption of ester prodrug derivatives of L-dopa, specifically, the alkyl, aryl, substituted and unsubstituted mono, di-and polyhydroxyalkyl and substituted and unsubstituted aralkyl esters thereof and their pharmaceutically acceptable counterion salts. Optionally, said compositions and methods may include a decarboxylase inhibitor such as carbidopa and benserazide. Thus, there exists a need for an absorbable form of L-dopa which can be administered by an alternative route (other than oral), and which results in therapeutically effective plasma levels of L-dopa.

Accordingly, a major object of the invention is to provide an absorbable form of L-dopa for rectal application.

Another object of the invention is to provide a prodrug form of L-dopa which is characterized by enhanced solubility, can be administered in standard pharmaceutical formulations to warm-blooded animals to elicit a systemic physiological or pharmacological beneficial effect, and which exhibits enhanced bioavailability and physiological availability.

Still another object is to provide L-dopa Prodrugs which will elicit a more effective sympathomimetic response, at lower concentrations or dosage levels, than its parent molecules.

The L-dopa esters of the invention can be administered rectally (with or without a decarboxylase inhibitor) as a suppository device or other modes administration. Release and absorption of the drug and subsequent hydrolysis in the blood would yield L-dopa which would be available to exhibit its intrinsic pharmacologic effects.

Other objects, features and advantages of the invention will be apparent to those skilled in the art from the description of the invention which follows.

DESCRIPTION OF THE INVENTION

The invention relates to compositions and methods of enhancing the rate of absorption of a rectally administered prodrug form of L-dopa. The compositions comprise an effective amount of an ester form of L-dopa (prodrug) and optionally a decarboxylase inhibitor such as carbidopa and benserazide formulated with commonly employed pharmaceutically acceptable excipients. The method generally comprises administering a dosage form capable of being rectally administered, wherein said dosage form comprises a therapeutically effective amount of an L-dopa ester in a sufficient quantity to be effective in enhancing rectal absorption rates and optionally a decarboxylase inhibitor. The L-dopa prodrug ester derivatives of the invention are best described by the general formula below: ##STR1## wherein R is a straight or branched chain alkyl (C₁ -C₂₀) such as methyl, ethyl, propyl, butyl, pentyl, tetradecyl, hexadecyl and the like; aryl(C₆ -C₉) such as phenyl, tolyl and the like; substituted and unsubstituted mono, di or polyhydroxyalkyl(C₁ -C₂₀) such as 4-hydroxybutyl, 2-hydroxypropyl, 2,3-dihydroxypropyl, 1,3-dihydroxypropyl, 6-hydroxyhexyl and 5-hydroxypentyl and the like optionally having a substituent such as alkoxy(C₁₋₅) [methoxy, ethoxy, butoxy and the like]; carbalkoxy (C₁₋₅) [methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl butoxycarbonyl and the like]; amino; mono or dialkylamino(C₁ -C₁₀) [methylamino, methylethylamino, diethylamino and the like]; acylamino(C₁ -C₅) [acetamido, butyramido and the like]; ketoalkyl (C₁ -C₅) [methylketo, ethylketo, butylketo and the like]; halo [chloro, bromo and the like] or carboxamide; substituted and unsubstituted aralkyl(C₇ -C₂₀) such as benzyl, alkoxybenzyl (C₈ -C₁₄) [methoxy, ethoxy, isobutoxy and the like]; phenylethyl; phenylpropYl; phenylbutyl; phenylhexyl; phenyloctyl and the like; and pharmaceutically acceptable organic or inorganic counterion salts.

The synthetic processes for preparing the esters of L-dopa and the salts thereof are known in the art and are hereby incorporated herein by reference, namely, U.S. Pat. Nos. 3,891,696 and 4,035,507 and Journal of Pharmaceutical Sciences, 62, p. 510 (1973). For rectal application, the formulations may be prepared as microenemas, suppositories, rectal tablets, rectal devices, sustained-release formulations, and other standard procedures known in the art. The preferred formulation is a solid suppository comprising a pharmacologically required dose of the prodrug that is sufficient to provide therapeutic plasma levels of L-dopa, optionally a sufficient quantity of a decarboxylase inhibitor, and sufficient suppository base to formulate an acceptable composition. The methods and choice of excipients and suppository bases are well known to those skilled in the art and the composition of said formulations is not limited to solid suppositories by this invention.

Generally, the amount of active agent employed in the practice of the invention ranges from 200 mg to 2.5 grams per day. Optionally, a decarboxylase inhibitor may be included at 1:25 to 1:4 ratio of the amount of active agent. The percentage of pharmaceutically acceptable excipients employed herein can range from 55% to 95% of the total composition weight.

The following examples illustrate preparation of various compositions of the invention. The examples should be construed as illustrative rather, than limitations thereof.

EXAMPLE 1

Aqueous microenemas (250 μl) containing either 2.0 mg L-dopa or 2.66 mg L-dopa ethyl ester HCl (equivalent to 2.0 mg L-dopa) were prepared. The solutions were adjusted to pH 5.5 and contained 0.02% ascorbic acid to prevent oxidation. Blood samples were collected following rectal administration to anesthetized rats, and plasma L-dopa determined by HPLC. The results, shown below, indicate significantly higher plasma levels of L-dopa were obtained with the L-dopa ethyl ester HCl formulation.

    ______________________________________                                                       Rat Plasma L-dopa (μg/ml)                                                   (mean ± S.D., n = 3)                                          Time of         Rectal     Rectal L-dopa                                       Plasma Sample (min.)                                                                           L-dopa     ethyl ester HCl                                     ______________________________________                                          0              N.D..sup.a N.D.                                                15              0.02 ± 0.02                                                                            0.27 ± 0.04                                      30              0.02 ± 0.01                                                                            0.20 ± 0.08                                      60               0.01 ± 0.001                                                                          0.08 ± 0.01                                      90              0.01 ± 0.01                                                                            0.05 ± 0.01                                      Percent L-dopa  1           12 ± 2.4                                        Bioavailability.sup.b                                                          ______________________________________                                          .sup.a N.D. = not detectible                                                   .sup.b Percent bioavailability calculated versus 2.0 mg Ldopa given by         i.v. injection                                                           

EXAMPLE 2

The experimental conditions of Example 1 were repeated, except that each formulation also contained 0.5 mg carbidopa, a decarboxylase inhibitor which is included to decrease L-dopa metabolism. The results, shown below, indicate a significant increase in plasma L-dopa levels and L-dopa bioavailability from the L-dopa ethyl ester HCl formulation (in the presence of carbidopa).

    ______________________________________                                                     Rat Plasma L-dopa (μg/ml)                                                   (mean ± S.D., n = 3)                                            Time of       Rectal     Rectal L-dopa                                         Plasma        L-dopa and ethyl ester HCl                                       Sample (min.) Carbidopa  and Carbidopa                                         ______________________________________                                          0            N.D..sup.a N.D.                                                  15            0.06 ± 0.03                                                                            0.76 ± 0.24                                        30            0.11 ± 0.07                                                                            1.00 ± 0.56                                        60            0.11 ± 0.05                                                                            0.60 ± 0.39                                        90            0.13 ± 0.10                                                                            0.36 ± 0.22                                        Percent L-dopa                                                                                 9 ± 5.5                                                                                62 ± 32.7                                        Bioavailability.sup.b                                                          ______________________________________                                          .sup.a N.D. = not detectible                                                   .sup.b Percent bioavailability calculated versus 2.0 mg Ldopa given by         i.v. injection                                                           

EXAMPLE 3

Aqueous formulations (pH 5.5, 0.1% ascorbic acid, 1.0 ml) containing 66.5 mg L-dopa ethyl ester HCl (equivalent to 50 mg L-dopa) were prepared for administration to Beagle dogs. Each dog received the rectal formulation following a 3-day pretreatment with oral carbidopa (25 mg b.i.d.). Blood samples were collected for 360 minutes and plasma L-dopa levels determined by HPLC. It is known that L-dopa itself is not significantly absorbed in dogs when administered by the rectal route. In the present studies, the bioavailability of L-dopa from the L-dopa ethyl ester HCl formulation was 34±25.4%, n=4. This represents a significant increase in Plasma drug levels due to the application of the ester described in the instant application.

EXAMPLE 4 L-dopa bioavailability in rats following rectal administration of L-dopa 2-hydroxyprooyl ester HCl

Each rat received 250 μl aqueous microenema (0.1% ascorbic acid, pH 5) containing either 2.0 mg L-dopa or 2.8 mg L-dopa 2-hydroxypropyl ester HCl in the presence or absence of 0.5 mg carbidopa. The solutions were administered at an intrarectal depth of 2.5 cm. Blood samples were collected for 90 minutes, plasma L-dopa determined, and percent L-dopa bioavailability calculated relative to intravenous L-dopa administration. The results are shown in the following table:

    ______________________________________                                                          Percent L-dopa bioavailability                                Administered     (mean ± SD, n = 3)                                         Solution         No Carbidopa                                                                               With Carbidopa                                    ______________________________________                                         L-dopa           3 ± 2    9 ± 6                                          L-dopa hydroxypropyl ester                                                                      .sup. 8 ± 3.sup.a                                                                       19 ± 9.sup.a                                   HCl                                                                            ______________________________________                                          .sup.a Statistically greater than Ldopa administration at the p < 0.05         level                                                                    

EXAMPLE 5 L-dopa bioavailability in rats following rectal administration of L-dopa 4-hydroxybutyl ester HCl

Each rat received a 250 μl aqueous microenema (0.1% ascorbic acid, pH 5) containing either 2.0 mg L-dopa or 3.1 mg L-dopa 4-hydroxybutyl ester HCl in the presence or absence of 0.5 mg carbidopa. The solutions were administered at an intrarectal depth of 2.5 cm. Blood samples were collected for 90 minutes, plasma L-dopa determined, and percent L-dopa bioavailability calculated relative to intravenous L-dopa administration. The results are shown in the following table:

    ______________________________________                                                          Percent L-dopa bioavailability                                Administered     (mean ± SD, n = 3)                                         Solution         No Carbidopa                                                                               With Carbidopa                                    ______________________________________                                         L-dopa           3 ± 2    9 ± 6                                          L-dopa 4-hydroxybutyl ester                                                                     100 ± 13.sup.a                                                                          100 ± 45.sup.a                                 HCl                                                                            ______________________________________                                          .sup.a Statistically greater than Ldopa administration at the p < 0.05         level                                                                    

EXAMPLE 6 L-dopa bioavailability in dogs following rectal administration of L-dopa 4-hydroxybutyl ester HCl

Dogs were Pretreated for 3 days with oral carbidopa (25 mg b.i.d.). Each dog, on the study day, received a 1 ml aqueous microenema (0.1% ascorbic acid, pH 5) containing either 50 mg L-dopa or 77.5 mg L-dopa 4-hydroxybutyl ester HCl. The solution was administered at an intrarectal depth of 4.5 cm. Blood samples were collected for 4 hours, plasma L-dopa determined, and percent L-dopa bioavailability calculated relative to intravenous L-dopa administration. The results are shown in the following table:

    ______________________________________                                                           Percent L-dopa bioavailability                               Solution Administered                                                                            (mean ± SD, n = 3)                                        ______________________________________                                         L-dopa            <1                                                           L-dopa 4-hydroxybutyl ester HCl                                                                  33 ± 14.sup.a                                             ______________________________________                                          .sup.a Statistically greater than Ldopa administration at the p < 0.001        level                                                                    

EXAMPLE 7 L-dopa bioavailability in rats following rectal administration of L-dopa ester

Various esters of L-dopa were rectally administered to rats and dogs at a dose equivalent to 2.0 mg L-dopa per kg body weight (rats) or 20 mg L-dopa equivalents per kg body weight (dogs). Plasma was collected and L-dopa determined by HPLC, and L-dopa bioavailability calculated versus i.v. administration.

    ______________________________________                                         L-DOPA BIOAVAILABILITY FOLLOWING                                               RECTAL ADMINISTRATION OF L-DOPA                                                AND L-DOPA ESTERS                                                                                  PERCENT L-DOPA                                                                 BIOAVAILABILITY                                            Administered        (mean ± SD)                                             Solution            RATS.sup.a                                                                               DOGS.sup.b                                       ______________________________________                                         L-DOPA              3 ± 2  ˜1                                         L-DOPA METHYL ESTER 46 ± 12                                                                               28 ± 5                                        L-DOPA ISOPROPYL    48 ± 17                                                                               30 ± 1                                        ESTER                                                                          L-DOPA HYDROXYPROPYL                                                                               8 ± 3  --                                               ESTER                                                                          L-DOPA BUTYL        100 ± 13                                                                              32 ± 4                                        ESTER                                                                          L-DOPA 4-HYDROXY-   13 ± 6  33 ± 14                                      BUTYL ESTER                                                                    ______________________________________                                          .sup.a Rats not pretreated with Carbidopa calculated versus i.v. LDopa         administration                                                                 .sup.b Dogs pretreated with oral Carbidopa (25 mg bid × 3 days)          calculated versus i.v. LDopa in carbidopa pretreated dogs                 

What is claimed is:
 1. A method of enhancing the rate of absorption of a rectally administered composition comprising a therapeutically effective dosage amount of an ester of L-dopa having the structural formula: ##STR2## wherein R=alkyl(C₁ -C₂₀); aryl(C₆ -C₉); substituted and unsubstituted mono, di or polyhydroxyalkly(C₁ -C₂₀); substituted and unsubstituted aralkyl(C₇ -C₂₀) and pharmaceutically acceptable organic or inorganic counterion salts thereof and a pharmaceutically acceptable amount of a decarboxylase inhibitor selected from the group consisting of carbidopa and benserazide
 2. A method of enhancing the rate of absorption of a rectally administered composition comprising a therapeutically effective dosage amount of an ester of L-dopa having the structural formula: ##STR3## wherein R=alkyl(C₁ -C₂₀), aryl(C₆ -C₉), substituted and unsubstituted mono, di or polyhydroxyalkyl(C₁ -C₂₀); substituted and unsubstituted aralkyl(C₇ -C₂₀) and pharmaceutically acceptable organic or inorganic counterion salts. wherein R=alkyl(C₁ -C₂₀), aryl(C₆ -C₉), substituted and unsubstituted mono, di or polyhydroxyalkyl(C₁ -C₂₀); substituted and unsubstituted aralkyl(C₇ -C₂₀) and pharmaceutically acceptable organic or inorganic counterion salts.
 3. The method of claim 1, wherein R is alkyl selected from the group consisting of methyl, ethyl, propyl, butyl and hexadecyl; aryl selected from the group consisting of phenyl and tolyl; substituted and unsubstituted mono, di or polyhydroxyalkyl selected from the group consisting of 4-hydroxybutyl, 2-hydroxypropyl, 2,3-dihydropropyl, 1,3-dihydropropyl, 6-hydroxyhexyl, 4-hydroxy-2-methoxybutyl, 5-hydroxypentyl and 2-hydroxy-4-diethylaminobutyl; or said substituted and unsubstituted aralkyl is selected from the group consisting of benzyl, o, m and p-methoxybenzyl and phenylethyl.
 4. The method of claim 3, wherein said alkyl is ethyl or isopropyl or butyl; said aryl is tolyl, said substituted and unsubstituted mono, di or polyhydroxyalkyl is 4-hydroxybutyl or 2-hydroxypropyl; and said substituted and unsubstituted aralkyl is phenylethyl or p-methoxybenzyl.
 5. The method of claim 4, wherein said alkyl is ethyl, isopropyl or butyl.
 6. The method of claim 4, wherein said hydroxyalkyl is 4-hydroxybutyl.
 7. The method of claim 4, wherein said aralkyl is phenylethyl and said aryl is tolyl.
 8. The method of claim 1, wherein said ester of L-dopa is in microenema or suppository form.
 9. A pharmaceutical composition for enhancing rectal absorption of L-dopa by administering a formulation comprising a therapeutically effective dosage amount of an ester of L-dopa of the structural formuala: ##STR4## wherein R=alkyl(C₁ -C₂₀); aryl(C₆ -C₉); substituted and unsubstituted mono, di or polyhydroxyalkyl(C₁ -C₂₀); substituted and unsubstituted aralkyl(C₇ -C₂₀) and pharmaceutically acceptable organic or inorganic counterion salts thereof and a pharmaceutically acceptable amount of a decarboxylase inhibitor selected from the group consisting of carbidopa and benserazide.
 10. The composition of claim 9, wherein R is alkyl.
 11. The composition of claim 10, wherein said alkyl is selected from the group oonsisting of methyl, thyl, propyl, butyl, pentyl, tetradecyl and hexadecyl.
 12. The composition of claim 11, wherein said alkyl is ethyl, isopropyl or butyl.
 13. The composition of claim 9, wherein R is aryl.
 14. The composition of claim 13, wherein said aryl is selected from the group consisting of phenyl and tolyl.
 15. The composition of claim 14, wherein said aryl is tolyl.
 16. The composition of claim 9, wherein R is substituted and unsubstituted aralkyl.
 17. The composition of claim 16, wherein said substituted and unsubstituted aralkyl is selected from the group consisting of benzyl, alkoxybenzyl (C₈ -C₁₄), phenylethyl, phenylpropyl, phenylbutyl and phenyloctyl.
 18. The composition of claim 9 wherein R is substituted or unsubstituted mono, di or polyhydroxyalkyl.
 19. The composition of claim 18, wherein R is an unsubstituted mono-hydroxyalkyl.
 20. The composition of claim 19, wherein said mono-hydroxyalkyl is selected from the group consisting of 4-hydroxybutyl, 6-hydroxyhexyl, 2-hydroxypropyl and 5-hydroxypentyl.
 21. The composition of claim 20, wherein said mono-hydroxyalkyl is 4-hydroxybutyl.
 22. The composittion of claim 20, wherein said mono-hydroxylalkyl is 2-hydroxypropyl.
 23. The composition of claim 18, wherein R is a dihydroxyalkyl.
 24. The composition of claim 23, wherein R is selected from the group consisting of 2,3-dihydroxypropyl and 1,3-dihydroxypropyl.
 25. The composition of claim 18, wherein R is a substituted mono-hydroxyalkyl.
 26. The composition of claim 25, wherein substituents on the hydroxyalkyl group are selected from the group consisting of alkoxy(C₁ -C₅), carbalkoxy(C₁ -C₅), amino, mono or dialkylamino(C₁ -C₁₀), acylamino(C₁ -C₅), ketalkyl(C₁ -C₅), halo and carboxamide.
 27. The composition of claim 26, wherein said alkoxy is methoxy, ethoxy or butoxy; said carbalkoxy is methoxycarbonyl or ethoxycarbonyl; said mono and dialkylamino is methylamino, methylethylamino or diethylamino; said acylamino is acetamido, propionamido or butyramido; said ketoalkyl is methylketo, ethylketo or butylketo; and said halo is chloro or bromo.
 28. The composition of claim 27, wherein said alkoxy is methoxy, said carbalkoxy is ethoxycarbonyl, said mono or dialkylamino is diethylamino, said acylamino is acetamido, said ketoalkyl is methylketo, and said halo is chloro.
 29. The composition of claim 9, wherein said decarboxylase inhibitor is carbidopa.
 30. The composition of claim 9, in microenema or suppository form. 